The present invention relates to filters for use in exhaust streams for capturing particulate material. In particular the present invention relates to porous ceramic diesel exhaust filters based on phosphate ceramics.
Co-pending U.S. Provisional Application having Serial No. 60/157,895 discloses a structure for use in filtration applications, such as diesel exhaust filtration, comprising predominantly an NZP-type phase having the general formula RxZ4P6xe2x88x92ySiyO24, where 0xe2x89xa6xxe2x89xa68, 0xe2x89xa6yxe2x89xa66, R is Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Y, and/or lanthanides, and Z is Zr, Ti, Nb, Ta, Y, and/or lanthanides, and optionally a sintering additive. The structure has an open porosity of at least 20% by volume, a median pore diameter in micrometers equal to or greater than a value defined by the quantity [10xe2x88x920.10(% open porosity)], both as measured by mercury porosimetry, and four-point modulus of rupture as measured on a solid rod of circular cross section, of at least 300 psi. This ceramic structure has a melting point in excess of 1700xc2x0 C. and a coefficient of thermal expansion lower than 10xc3x9710xe2x88x927/xc2x0 C., making it extremely suitable for high temperature applications such as diesel particulate filters.
Notwithstanding these findings, reaction sintered NZP-type phase ceramics of the type described when extruded into large size (i.e., greater than 4xe2x80x3 in diameter) cellular honeycomb structures have a tendency to crack during/after binder removal, but before sintering due to the large particle size of the raw materials.
A need therefore exists for honeycomb cellular structures comprising reaction sintered NZP-type phase ceramics which remain wholly intact and crack-free despite using large raw material sizes.
The present invention provides such a honeycomb cellular structure and a method of making the same.
The present invention provides a method of making a ceramic filter, which includes the steps of forming a mixture from NZP-forming raw material powders selected from the group consisting of metal oxide sources capable of reacting to form a reaction product comprising an NZP-type phase having the general formula R1+(y/2)Zr4P6xe2x88x92ySiyO24 where 0xe2x89xa6yxe2x89xa61.0 and R is one or more of the metals Ca, Sr, and Ba, pre-reacted powder having the same general formula, and mixtures thereof; and a precursor additive selected from the group consisting of silica precursor, zirconia precursor, and mixtures thereof; shaping said mixture into a green structure; and, firing said green structure to produce a ceramic filter. The precursor additive is at least 1% based on the weight of the mixture, and more preferably between 4% and 8% based on the weight of the mixture.
The mixture may also optionally include a pore former, such as graphite, of at least 10% based on the weight of the raw materials and a sintering aid selected from the group consisting of magnesium, zinc, calcium, aluminum, lanthanum, titanium, bismuth, tungsten, and mixtures thereof. The sintering additive is between about 0.05 wt. % to 10 wt. % based on the weight of the raw materials.
The inventive method is especially useful in enabling the production of large-size honeycomb cellular substrates having than 4 inches in diameter which do not crack during the firing (sintering) cycle. The cellular substrates are particularly suitable as diesel particulate filters and have an open porosity of at least about 35% by volume, a median pore size of at least 8 micrometers, and a permeability of greater than 0.30xc3x9710xe2x88x9212 m2.